Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") comprising a solid polymer electrolyte or ion exchange membrane disposed between two fluid distribution (electrode substrate) layers formed of electrically conductive sheet material. The fluid distribution layer has a porous structure across at least a portion of its surface area which renders it permeable to fluid reactants and products in the fuel cell. The electrochemically active region of the MEA also includes a quantity of electrocatalyst, typically disposed in a layer at each membrane/fluid distribution layer interface, to induce the desired electrochemical reaction in the fuel cell. The electrodes thus formed are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fluid fuel stream moves through the porous portion of the anode fluid distribution layer and is oxidized at the anode electrocatalyst. At the cathode, the fluid oxidant stream moves through the porous portion of the cathode fluid distribution layer and is reduced at the cathode electrocatalyst.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O
In electrochemical fuel cells employing methanol as the fuel supplied to the anode (so-called "direct methanol" fuel cells) and an oxygen-containing stream, such as air (or substantially pure oxygen) as the oxidant supplied to the cathode, the methanol is oxidized at the anode to produce protons and carbon dioxide. Typically, the methanol is supplied to the anode as an aqueous solution or as a vapor. The protons migrate through the ion exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations: EQU Anode reaction: CH.sub.3 OH+H.sub.2 O .fwdarw.6H.sup.+ +CO.sub.2 +6e.sup.- EQU Cathode reaction: 3/2O.sub.2 +6H.sup.+ +6e.sup.- .fwdarw.3H.sub.2 O
In electrochemical fuel cells, the MEA is typically interposed between two separator plates or fluid flow field plates (anode and cathode plates). The plates typically act as current collectors, provide support to the MEA, and prevent mixing of the fuel and oxidant streams in adjacent fuel cells, thus, they are typically electrically conductive and substantially fluid impermeable. Fluid flow field plates typically have channels, grooves or passages formed therein to provide means for access of the fuel and oxidant streams to the surfaces of the porous anode and cathode layers, respectively.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate serves as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell, hence the plates are sometimes referred to as bipolar plates. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack. The stack typically includes manifolds and inlet ports for directing the fuel and the oxidant to the anode and cathode fluid distribution layers, respectively. The stack also usually includes a manifold and inlet port for directing the coolant fluid to interior channels within the stack. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant streams, as well as an exhaust manifold and outlet port for the coolant fluid exiting the stack.
The fluid distribution layer in electrochemical fuel cells has several functions, typically including:
(1) to provide access of the fluid reactants to the electrocatalyst; PA1 (2) to provide a pathway for removal of fluid reaction product (for example, water in hydrogen/oxygen fuel cells and water and carbon monoxide in direct methanol fuel cells); PA1 (3) to serve as an electronic conductor between the electrocatalyst layer and the adjacent separator or flow field plate; PA1 (4) to serve as a thermal conductor between the electrocatalyst layer and the adjacent separator or flow field plate; PA1 (5) to provide mechanical support for the electrocatalyst layer; PA1 (6) to provide mechanical support and dimensional stability for the ion exchange membrane. PA1 (a) carbon fiber paper; PA1 (b) woven and non-woven carbon fabric--optionally filled with electrically conductive filler such as carbon particles and a binder; PA1 (c) metal mesh or gauze - optionally filled with electrically conductive filler such as carbon particles and a binder; PA1 (d) polymeric mesh or gauze, such as polytetrafluoroethylene mesh, rendered electrically conductive, for example, by filling with electrically conductive filler such as carbon particles and a binder. PA1 (e) microporous polymeric film, such as microporous polytetrafluoroethylene, rendered electrically conductive, for example, by filling with electrically conductive filler such as carbon particles and a binder. PA1 (a) a pair of substantially fluid impermeable separator plates; PA1 (b) a pair of fluid distribution layers interposed between the separator plates, each of the fluid distribution layers having two major planar surfaces, at least one of the fluid distribution layers comprising a sealing region and an electrically conductive, fluid permeable active region, the at least one fluid distribution layer comprising a preformed sheet material extending into each of the sealing region and the active region; PA1 (c) an ion exchange membrane interposed between at least a portion of the fluid distribution layers; PA1 (d) a quantity of electrocatalyst interposed between at least a portion of each of the fluid distribution layers and at least a portion of the membrane, thereby defining the active region. PA1 (a) a pair of substantially fluid impermeable separator plates; PA1 (b) a pair of fluid distribution layers interposed between the separator plates, each of the fluid distribution layers having two major planar surfaces, at least one of the fluid distribution layers comprising a sealing region and an electrically conductive, fluid permeable active region, the at least one fluid distribution layer comprising a porous electrically insulating sheet material extending into each of the active region and the sealing region; PA1 (c) an ion exchange membrane interposed between at least a portion of the fluid distribution layers; PA1 (d) a quantity of electrocatalyst interposed between at least a portion of each of the fluid distribution layers and at least a portion of the membrane, thereby defining the active region.
The fluid distribution layer is electrically conductive across at least a portion of its surface area to provide an electrically conductive path between the electrocatalyst reactive sites and the current collectors. Materials which have been employed in fluid distribution layers in solid polymer electrochemical fuel cells include:
Thus, fluid distribution layers typically comprise preformed sheet materials which are electrically conductive and fluid permeable in the region corresponding to the electrochemically active region of the fuel cell.
Conventional methods of sealing around MEAs within fuel cells include framing the MEA with a resilient fluid impermeable gasket, placing preformed seal assemblies in channels in the fluid distribution layer and/or separator plate, or molding seal assemblies within the fluid distribution layer or separator plate, circumscribing the electrochemical active region and any fluid manifold openings. Examples of such conventional methods are disclosed in U.S. Pat. Nos. 5,176,966 and 5,284,718. Disadvantages of these conventional approaches include difficulty in assembling the sealing mechanism, difficulty in supporting narrow seal assemblies within the fluid distribution layer, localized and uneven mechanical stresses applied to the membrane and seal assemblies, and seal deformation and degradation over the lifetime of the fuel cell stack.
Such gaskets and seals, which are separate components introduced in additional processing or assembly steps, add complexity and expense to the manufacture of fuel cell stacks.